Moisture measuring apparatus



Sept- 24,.1963 w. R. BLYTHE ETAL 3,105,214

l MOISTURE MEASURING APPARATUS File d Feb. 25. 1959 6 Sheets-Sheet v1 Y LEXANDl-'R KlE//V Sept. 24, 1963 w. R. BLYTHE ETAL MOISTURE MEASURING APPARATUS 6 Sheets-Sheet 2 Fla. 5

f Rm W ovl/ N m B MR. n MD .A N m4 WA 0 l w G. I

O w wp .IMD But ING em .we mmm e RW O O O O O O 4 O. O. O. O. D O 5 4 mv 2v 1 s, mwa .nog OVJd gum MB ,Sept- 24, 1963 w. R. BLYTHE x-:TALY A 3,105,214

. MOISTURE MEASURING APPARATUS e sheets-sheet :s

Filed Feb. 25. 1959 BIvegol'nns ou oucos b|^ ildge 2 0@ "'15 inches long. 1.00 BO Inches long. O

40 I 60 8O 1GO Relative humidity percent.

Fm. li

'BY amm' Almen/YY SePf- 24, 1963 w. R. BLYTHE r-:TAL 3,105,214

MOISTURE MEASURING APPARATUS Filed Feb. 25, 1959 6 Sheets-Sheet 4 2.4 Megohms, Bouyoucos bridge. 213

22 FIG. l2 21 40 50 60 70 80 Q0 100 Relative humidity, percent.

i 2.3 Megoh ms, Bouyoucos 2 2 brid ge. l 2.1 l

Relative humidity, percent,

l0 turns per inch. -20turns per inch. Me ohms, Bou/oucos 2,00 i bridge. 1

\ mvENToR.

w/L/AM R. B/.Yruf y ALEXAA/Df/z KLf/N 56. I4 0 I BY l v4o 6o 8o ioo MZ Relative humidity percent. Z'JWKZVFY cos bridge.

Sept. 24, 1963 W. R. BLYTHE ETAL 3,105,214 MOISTURE MEASURING APPARATUS Filed Feb. 25. 1959 6 Sheets-Sheet. 5

10.0 vciporcible 'moisture interior sample.

content, percent 8.0

dry weight.

Whole cylinderJ Weight loss, percent 0f one doy weight.

| lO turns per inch 20tu rns per inch 40 60 8O lOO 40 60 8O l OO HGISRelcitive humidity, percent. [-76.16 Relative humidityl percent.

Megohms, Bouyoucos bridge.

Megoh ms, Bouyou 2.00 Megohms, Bouyoucos bridge. vinyl cicetcite copolymer.

polyvinyl cilconol. O f l` l Fla/7 0 2 4 e a im, l0

HCl-IPS. wuz/AM ngi/915A? ALMA/vof@ /fLf/A/ -BMMAW Sept. 24; 1963 w. R. BLYTHE ETAL A1*,105214 MOISTURE MEASURING APPARATUS Filed Feb. 25. 1959 6 Sheets-Sheet 6 l, 3.2 k Illcre'clsleJL l ln wel percelt 3.0 \l/ l of orehcoy N f wel g 28 ,l/ Y

v units without membranes. units with permselectlve membranes. 2.4

O 100 ZOO BOO 400 5OO Ohms. Bouyoucos bridge.

. Weight 7.0

loss, specimens stored at 100F& 20% RH. percent 60 of one doy weight.

5.0 40 /gpecimens stored at 70F 50l 3.o

BY www United tates 3,105,214 MGETURE MEASURlNG APPARATUS William it. Blythe, Mountain View, and Alexander Klein, lanviile, Caiit., asngnors to The Regents of The Uur1 versity ot' California, Berkeley, (Calif.

Filed Feb. 25, 1959, Ser. No. 795,557 i3 Claims, (Cl. E38-35) This invention relates to the measurement ot moisture and to moisture meters and probes therefor. More particularly, it relates to the measurement of moisture without relation to effects due to changes in mobile-ion concentration. Still more particularly, it relates: to an electrical moisture meter and .to a probe therefor unaiected by salinity in a porous: medium whose moisture content is being measured. This application is a continuation-inpart of application Serial Number 704,944, tiled December 24, 1957, now abandoned.

The electrical properties, including resistance and capacitance, of many materials vary according to the amount of water in the material, and this variation has served as the basis for many kinds of moisture meters. For example, moisture measurements have been based upon the change in electrical resistance or capacitance of a porous medium caused by changes in moisture content of the medium. The porous medium may be the very body whose moisture content is being. measured (as where bare electrodes are embedded in soil or concrete) or may be separate blocks containing the electrodes and inserted in the body being measured, coming to an equilibrium moisture-content status with respect to its surroundings (where a Bouyoucos plaster-of-Paris block is embedded in soil). In either case, assuming that the dry material is nonconductive, the electrical resistance of the porous rnedium is .a function of (1) the amount of water in the medium through which the electric current travels and (2) the numlber of mobile ions present in the water. Thus, the resistance will vary inversely with (l) water content and (2) salinity. In addition, the resistance or water increases with decrease in temperature. Also, when the water content is expressed as percent of the dry Weight (as is customary in expressing the moisture content of soils), the same percent of water content means more water per unit uolume with a denser material than with one less dense.

Eitects of density and temperature changes on the calibration of an instrument can generally be measured and taken into account. Changes in the salinity of pore water, on the other hand, are not easily controlled or measured. In fact, none of the electrical moisture gauges heretofore known were capta-ble of determining the moisture contents of concrete to the accuracy needed for adequate understanding of the eitects of that moisture. Nor was any such gauge capable of determining moisture gradients in relatively .thin sections. The failures of these prior-art gauges have been due largely to the eitects of salinity and to the impossibility of eliminating these elfects yfrom the prior-art devices. The change in resistance, which supposedly was ya measure of the change in moisture content, was affected both by the nature of the ions and by their concentration, introducing serious inaccuracies` into the measurement oi moisture in soils, mortar, and concrete, for example, as well as elsewhere. Wherever soluble salts are present, they'ailect the electrical measurements considerably. Moreover, the salt content mlay change with time (due to leaching, fertilizing, and other factors) so that even if it were known at any one time, accurate compensation could not be made for the subsequent measurements.

For example, when copper-plated carbon electrodes are embedded directly in soil and an alternating current is applied (as was `done iby the United States Department odi 3,165,214 Patented Sept. 24, 1963 Agriculture as early as 1897), variations in moisture content to produce changes in resistance that can be measured on a Wheatstone bridge. However, the resistance varies also according to the salt content of the pore water. Also, the physical contact of the electrodes with the soil varies as the soil dries and shrinks, and this may also cause considerable undesirable eiects.

An attempt was made to solve the contact problem by embedding the electrodes in a controlled environment rather than directly in the body being measured. For example, electrodes have been cast in a small .block of plaster yof Paris; the porous block absorbs moisture and comes to an equilibrium with its surroundings'. Similarly, nylon, mortar, glass fibers, and other materials have been used for this purpose. But none of them both register the eects of moisture and remove the etiects of salinity on the pore water. lin all instances, what they may indicate at high moisture content may simply be a high concentre-.tion of ions in a relatively small amount of moisture.

To give an idea of the extent of .the inaccuracies to which prior-art moisture meters were subject, one investigator found that the electrical resistance of concrete varied not only with moisture content, but also with temperature, mix proportions, and composition diiterences in Portland cements and concretes. (R. W. Spencer, Measurement of Moisture Content in Concrete, Journal of the American Concrete Institute, vol. 34, pp. 45-6'1, 1937.) He .found that changes in the concentrations of soluble salts sometimes accounted for as much as a 40 percent change in resistance values. He found that the loss of other soluble materials, due to leaching by ,water moving through the mass, also had considerable eilect upon resistance Values. Variables dependent upon the mix `and upon the curing conditions were evaluated by Spencer by casting and sealing control electrodes in glass jars and placing them within the structure near .the field electrodes. Resistance readings of different sets of electrodes varied as much as 50 percent due to chance location of coarse pebbles close to or between the electrodes. Other limitations, listed by Spencer, included (1) failure of the resistance measurements to give la reliable indica-y tion of increases -in moisture content after moist curing, over that originally present due to the mixing water, and (2) increased resistance readings due .to poor bonding of the electrodes caused by shrinkage in the concrete.

Strain measurements of mortar specimens have been related to the moisture content of the specimens and with fair accuracy. However, several days are sometimes required before equilibrium is reached, and the method is not suitable for all porous media. Further, strain gauges are useless in structures under load or otherwise stressed.

Some other methods that have ibeen somewhat successful when used in large masses `of concrete employ equipment that is inherently so large that they are not adaptable to the investigation of moisture distribution thin sections.

One object of the present invention is .to provide -a moisture meter with a probe which at a given temperature is affected .only by moisture 'and not by the ionic activity within the medium whose moisture content is to be measured.

Another object of the invention is to provide a moisture-measuring device capable of relatively rapid response to moisture changes w-hile still barring the ionic eiects of salinity in the material whose moisture conditions are studied. l

Another object of the invention is to provide a relatively inexpensive, simple moisture-measuring probe which may be left permanently in place in concrete structures or in soil or in other installations to indicate the moisture therein whenever measurement is desired.

small enough to measure moisture gradients in relatively thin sections.

Another object of the invention is to provide a device which can be calibrated at various mosture contents in' terms of other properties of the body being measured, such as soil density or concrete density, and can be used to indicate changes in these properties.

In general, a typical article embodying the invention may be described as a probe comprising a pair of conductors, in themselves not moisture sensitive but with moisture .sensitive material between them, encased in a permselective ionic barrier which passes moisture adequately free of electrolytes. The barrier may be one or more resinous, ion-selective, semipermeable membranes The invention, however, includes reiinements of this general concept and of the method involving it. It alsoI incorporates a wide range of structures and materials used in various manners. f

Other objects and advantages of the invention will appear from the following description of some preferred 'embodiments thereof. Y

In the drawings:

FIG. 1 is a perspective View of a moisture meter probe embodying the principles of the present invention.

v FIG. 2 is a view similar to FIG. l showing the probe with the ionic barrier removed.

FIG. 3 is an `exploded View of the probe of FIG. l.

FIG. 4 is an enlarged view in section taken along the line 4-4 in FIG. l. Y

FIG. 5 is a plan view, partly in section, of a modified form of the invention, employing a printed circuit on a ilat base, shown with various layers successively broken away.

FIG. 6 is an enlarged view in section of the `complete probe of FIG. 5, taken along the line 6 6 of FIG. 5.

FIG. 7 is a view inyelevation and partly in section of another modified form of moisture meter probe embodying the invention, with portions of its outer ionic-barrier covering broken away to show the interior.

FIG. 8 is a view in section taken along the line 8 3 in FIG.. 7. f

FIG. 9 is a circuit diagram showing the moisture meter probe of `FIG. `1 connected to a measuring bridge (Bouy- Oucos bridge) utilizing an oscillating current. y

FIG. l0 is a graph showing a typical response obtained when using the bridge circuit of FIG. 9 in atmospheres of various known relative humidities, thereby indicating a Calibrating curve for relative humidity.

FIG. l1 is a graph showing the differences in response between two meters both having probes wound at twenty turns per inch, one fifteen inches long and one thirty inches long.

FIG. 12 is a graph of two curves showing the response of two units at 70 F. both with probes wound at twenty l turns per inch and coated with a dilute vinyl acetate copolymer, plotting megohm measurement against relative humidity.

FIG. 13 is a graph similar to FIG. 12 differing only in that a more concentrated solution or the same polymer was used.

FIG. 14 is a graph like that of FIG. l2 for two instruments in which a dilute solution of polyvinyl alcohol was used as the hygroscopic coating medium. One instrument probe was wound at ten turns per inch, and the other one was wound at twenty turns per inch.

FIG. 15 is a view like FIG. 14 for similar instrument probes in which a more concentrated solution of polyvinyl alcohol was used.

FIG. 16 is a graph plotting relative humidity against resistance in megohms fortwo instrument probes differing only in the concentration of the coating solution used, showing the effect of concentration of the coating solution on a single graph.

Vl5 and may also comprise the moisture-sensitive material. Y

FIG. 17 -is a graph showing the time required for two typical hygrometers without permselective membranes to achieve equilibrium. The two curves represent results v from two different coating materials, time being plotted against resistance yfor measurement of humidities of known Values.

FIG. 18 is a graph plotting evaporable moisture content of mortar samples against weight loss of the samples, one a whole cylinder, the other an interior sample.

FIG. 19 is a graph plotting weight loss against drying time of specimens stored at -two different conditions of temperature and relative humidity.

FIG. 20 is a graph plotting resistance against weight change in individual units subjected to leaching and comparing meters having permselective membranes with meters not having these membranes. v

FIG. 1 shows one form of moisture meter probe 39 embodying the present invention. This probe 30 has a small cylindrical body 3l. Typical sizes vary from less than one to more than ten inches long and from about one-quarter inch to one inch in diameter, though the body 31 may readily be made larger or smaller, if desired. From one end ofthe body 31 two leads 32. and 33 extend, and they may be connected to any suitable electrical metering device, of which the bridge shown in FIG. 9 is only an example. The resistance across the leads 32 and 3.3 may be measured and calibrated in terms of moisture conditions in any medium wherein the body Il is inserted. Electrical impedance across the leads l32 and4 33, including not only resistance but capacitance and both together, may similarly be measured and calibrated in lterms of moisture conditions. Because of the wide variety of sizes feasible, the probe 30 may be used in very small samples to study moisture gradients therein.

FIGS. 2-4 illustrate the construction details of the probe 3l). This particular unit employs a core 34 of .suitable dielectric material. Preferably, the core 34 should be of extremely low electrical conductivity. Glass appears to introduce a hysteresis eifect due to adsorption of water when its surface is etched to provide grooves, unless its surface is covered with a suitable waterproof material such as polystyrene. Preferably the core 34 is made directly from polystyrene tubing or other similar material.

The core 34 is provided with shallow bifilar grooves 35 and 36, as by machining in la metal lathe. [In the grooves 35 and 36 may be wound respective wires 37 and 38. The grooves 35 and 36 are shallow, because the wires 37 and 38 preferably lie on or very close to the surface of the core 34, rather than being Vset down into it. The ends of the wires 37 and 38 may be sealed to the core 34 by .a soldering iron or by cement. Wires which are corrodible by moisture are obviously undesirable, because errors may be introduced into the measurements by the formation of a r'ilm of oxide or salt. So noble metals and non-fll-m-forming alloys are preferable materials for the wires 37 and 38. Gold, platinum, and palladium wires are suitable, .as are wires plated with such metals. For example, jewelers" palladium vvire, such as Baker alloy No. 839, which is 96% pure palladium and No. 38 AWG (American Wire Gauge), may be used.

When the wires 37 and 3S have been wound on the core 34, the wires .and core may be coated with a moisture-sensitive film 40, which is often hygrosccpic. Any Afilm sui-table Afor use in hygrometcrs may be used, ie., any material that absorbs 'water vapor and `comes to an Vequilibrium moisture state with respect to its environment or surrounding atmosphere in a conveniently short time. 'For example, polyvinyl alcohols, some of Iwhich are sold under the trademark E-lvanol by E. I. du Pont de Nemours, are preferred for many uses. Some polyvinyl iacetates and chlorides are also good. Plaster-of-Paris may be used, but is bulkier, more fra-ngible, and more likely to deteriorate with time. -Nylon and many other materials areV usable, although for any particular set of f conditions some coatings may be preferable to others. As will be seen later, materials incorporating ionexchange resins yas a component may be employed as the moisture-sensitive film Atti. in fact, the ionic barrier itself may serve `as fthe moisturesensitive film 24d.

The film 4h may be applied by dipping the wire-wound core into .a solution or suspension of the `hlm material or into molten iilrn material, or the film di?` may be sprayed ton or brushed on or otherwise applied. For example, polyvinyl `acetate may be dissolved in ethanol or in a mixture of ethanol and distilled Water; partially hydrolized polyvinyl alcohol may be dissolved in Ia mixture of distilled water and ethanol. As another example, ion-exchange resins may be suspended in polyvinyl acetate, polyvinyl chloride, or polyethylene. The core may be dipped in such solutions, drained, and dried. More dilute solutions may be sprayed. The more :dilute the solution, the thinner `a uniform coating tha-t may be obtained. The thickness of the 4G and its composition may be varied to Igive desired results, as will be explained later.

The article 4l resulting `from winding the wires 37 and 3S on the core 34 and then applying the coating liti' is shown in PIG. 2 and yis a hygrometer or hygrostat. It may be immersed, encased, tor embedded in various things for measuring their moisture content, but it is subject to the eects of salinity as well as of moisture, except where ione-exchange resins are a substantial ycomponent: of the coating 4b. In the present invention the eects of salinity are avoided by complete encasement of the hygrometer il in a per-mseleotive case or coating 42 that forms an ionic barrier.

The case i2 may be one or more resinous, ion-selective, semipermeable membranes. There are many permselective materials. For example, collodion passes water vapor but not liquid water. Amon-g the most useful for this invention are ion-exchange resins. Some iof these are incorporated in membranes produced sheet form; some may be incorporated in ya form suitable for `dipping. The membranes readily allow passage of ions of one `charge while offering great resistance to ions of the opposite charge. Accordingly they are known as cation permeable membranes and anion permeable membranes. 'Ihose that are cation-permeable :are anion-impermeable, and those that are anion-permeable are cationimpermeable.

One typical cation-perrneable membrane may be regarded as a three-dimensional network of an insoluble organic polymer. Bound into the chains which make up this network are reactive chemical groups such as sulfonic, carboxylic, or phenolic. The interstices between the chains are filled with water, and the attached `acidic groups, being more or less free to dissociate, `are capable of exchanging any cations which may be in the water in the interstices. Random passages exist through the membranes which, in the ideal case, are of such Width that ions can pass through only by displacing some of the ions on the acidic groups lining the passages. Such `displacement can occur Ionly by an ion exchange mechanism. Since acidic groups lining the passages of a cation-permeable membrane can exchange only cations, the membrane is readily permeable to cations but ofers great resistance to the passage of anions.

YOn the other hand, basic groups line the passages in an lanion-permeable membrane, and these exchange only anions. The anion permeable membrane is therefore readily permeable to `anions but offers resistance to the passage of cations. y

There is nothing critical about which permselective .membranes are used, though some are preferable under cer-tain circumstances. A discussion of various materials may be found in Eodamer Patents Nos. 2,681,319 'and 2,681,320. But other perm-selective membranes may be used. The use of both .anion-permeable and cation permeable membranes gives ldouble protection but is not d `always necessary. In many instances, an anion-permeable membrane alone is suiiicient, and in many instances a cation-permeable membrane alone is sufficient.

FIG. 3 shows a at, flexible anion-permeable sheet 43 tha-t is wound around the hygromet-er probe 41 next to the coating 4d. Discs *44 and `45 of the same material are used at the ends of the 4core 14, `and are sealed to the cylindrical sheet 43 by heat or cement, to eliminate passage of moisture to the hygrometer il -except through the anion-permeable material 43, 4d, and 45. if desired, a catien-permeable sheet 45 and catio-n-permeable vdiscs 47 and 4g may be placed around the sheets 43, 44, `and 45. The discs 6,7 :and 48 tare sealed to the sheet 46. If desired, the cation-permeable membranes may be placed inside .and the lanion-pern'ieable membranes outside. For some cases, only one membrane of either type is enough. in place of this type `of assembly, the hygrometer probe il may be dipped in a liquid `form or solution or suspension of permselective membrane ingredients. In either event the wires 37 and 38 are connected respectively to the insulated leads 32. `and 33, .and the insulation is sealed to the permselective membranes.

The complete probe 3i), then, has a permselective casing 42 that permits the passage of water vapor while barring the passage of ions. Water vapor passes to the moisturesensitive film or coating 4t) until an equilibrium condition is reached with respect to the environment. If the environment outside the permselective coating 42 becomes dried, the coating in will give up water, and that water will pass back through the permselective membrane 42. In other words, water may pass both ways; so an equilibrium condition is maintained with some time lag. The extent of the time lag depends on the material used in the coating 20 and on the permeability by water vapor of the casing 42. These both may be chosen to give relatively quick effects. The device may be made to respond Within a few minutes, or several hours may be necessary with some constructions.

The Water vapor that is absorbed by the coating 4i) greatly alters the electrical resistance between the bifilar wires 37 and 38. The coating di), when dry, is substantially non-conductive, but as water vapor is absorbed, it becomes more and more conductive. As a result, a measurement of the resistance or impedance by a suitable type of iohmmeter or other device will give a curve which can be calibrated, ias will be explained later, to correlate resistance values or impedance values with relative humidity conditions or with the moisture pervading the environment in which the body 3l is placed.

A modified form of the invention -is shown in FGS. 5 and 6. Here the moisture sensing unit or probe Si) is flat instead of cylindrical. The base S1, which replaces the core 34, may be simply paper, preferably coated with plastic, or may be a plast-ic film, such as polystyrene. In this particular instance the unit is shown with a printed circuit comprising two printed filaments 52 and 53 corresponding to the wires 37 and 3S and spaced apart uniformly over their length. They may be on the same side of the base 5l, as shown, or on opposite sides. A hygroscopic or moistureasensitive medium S4 may be printed on the iilm base 51 or otherwise applied as by dippinI spraying, brushing, or other means, to fill the spaces between the two conducting elements 52 and 53 with the necessary moisture-sensitive coating. The result of these actions is again to provide a hygrome-ter )or probe 55, which is ilat, but in other ways generally resembles the probe '4l and operates in substantially the same manner. The coating may be polyvinyl alcohol as before, or may be any other suit-able medium.

Then, as previously, the hygrometer 55 is coated or encased in a permselective casing 56. This casing may be applied by folding kand sealing a sheet or sheets of permselective material over the hygrometer 55, or the hygrometer `55 may be dipped or sprayed to cover it with `a sufficient thickness of permselective material. In case the coating 54 requires air around it, a thin polar sheet such as lens paper 57 may be inserted to provide the presence of an effective vapor chamber 58. The operation of the unit 50 is substantially identical to that discussed inY connection with the meter 30 shown in FIGS. 1 through 4.

FIGS. and 6 also illustrate that instead of having the sensing element cylindrical, it may be plate-l-ike or waferlike, and that the conducting elements may be wire, plates, or printed elements with the space between them again being a suitable moisture-sensitive material.Y This helps to illustrate the breadth of the present invention.

While cylindrical forms are not necessary, it may be mentioned that it is one way of providing uniform results where Wires are used and insuring that the wire will be embedded evenly and completely in the coating. Printed' circuits may also be made uniform, but when hygrometers are formed by winding on square or rectangular forms, there is usually some overlap at the edges where the turns cannot be made to precisely lit the form, and therefore the coating is not even. Thesev units may be used, but they are not considered to be as reliable as the others that have been described where relatively short conductors are employed; in large meters the effect of overlap may beinconsequential. Flat, leaf-like wire-wound forms ure also highly practical.

An important feature of the invention is its ability to employ the permselective membrane itself in several capacities--as the core or as the moisture-sensitive medium as well as the ionic barrier coating, or as all of them. Remembering 'that this membrane may be either an already-made sheet 4or may be made in situ by dipping, spraying, or brushing, and that one or more ion-exchange resins, for example, may be used either alone or in a su-itable suspending medium, the invention has great adaptability. Thus, the core 34 -or 5l may be an ion-selective membrane,v and the coating 40 or 52. may be formed by dipping or spraying an ion-exchange resin on the conductors and core. The conductors may be wound on or embossed directly on -a membrane-type core. When the coating 46 or 52 is itself an ion-.exchange resin, no additional permselective membrane is needed; the resin already present functions in a dual capacity. Just as in the probes as heretofore described, such probes are Vembeddable in bodies having electrolytic components and the electrolytes will not affect operation.

FIGS. 7 and 8 show a unit 6@ generally resembling the unit 3) but with some important differences. Here a core 6l (which may be a permseleotive membrane) is provided with bilar grooves located in two seriesor groups 62 and 63. One group 62 is much longer than the other group 63 and for purposes of the example, the groove spaeings are identical, although they may be different in a particular unit if desired. In the group 62 two wires 64 and 65 are wound as the wires 37 and 33 were wound,

and two other wires 66 and 67 are similarly wound in the group `63. The wires 64, 65, 66, and 67 are respectively connected to leads 68, 69, 70, and 7l, preferably of a plastic-insulated type. VThus, the group 62- comprises a hygrorneter like the unit 30 but somewhat longer and therefore with Va lower rover-all resistance. The group 63 is @also like the unit 30 but is shorter and, of course, is much shorter than the unit 62, so that its resistance is substantially higher than theirs. The groups 62 and 63 are then coated with \a moisture-sensitive medium 72, which may be polyvinyl alcohol, an ion-exchange resin, Aor other suitable material, to provide two hygrometers 73 and 74. The hygrometers 73 and 74- are then coated with permselective membrane 75, as before (unless the ,membrane 75 and medium72 are unitary), to provide an ion barrier that permits the passage of moisture in the form of water vapor while denying passage to ions of electrolytes.

The unit 66 has a variety of uses. The two groups 62 and 63 may be used separately, in parallel or in series, so

that it is possible, by connecting the leads 63, 69, 7i), and '7l to a meter Yin various ways, to obtain from this one uni-t 6u four dilferent readings. A simple four-position electricy switch maybe provided in the measuring bridge to simplify operations, so that :any one position may be obtained simply by moving the switch. This means that calibrations under various circumstances can be obtained to give curves which will vary somewhat from each other though they will have general similarities. Such a unit can make very accurate moisture readings.

As stated earlier, practically any type of electrical ohmmeter may be used to obtain the resistance between the bilar windings or sensing conductors and the moisturesensitive coating that separates them. Purely for the sake of example of illustrating a suitable bridge, the electrical circuit of FIG. 9 is shown. It is not by Iany means necessary to use this particular circuit although it is an excellent one and will give very good results'.

The power for the Vcircuit of FIG. 9 is preferably obtained from an oscillator circuit. Filament voltage may be obtained from a 11/2Y volt A battery 30 while the plate voltage may be obtained from a 45-volt B battery 8l. The battery Sil is connected by leads S2 and 83 to filaments 84 zand 85 of a pair of tetrode tubes S6 and 87. In both tubes 86 and S7, the respective screen grids 88 and S9 are connected by lead 9@ to the positive side of the' battery 3l, the lead 9@ .to the screen grid 8S leading through the primary winding 91 of la transformer 92.

In the tube 66 the grid 93 is biased relative to the lilament 34 by a resistance 94,and is connected through a condenser 95 to a tuned parallel inductance 96 land condenser 96a, the other side of the network being connected to the lead 83. The inductance 96 is also the secondary winding of the transformer 92.

rllhe'lead 9G is connected across a load resistance 97 to the plate 93 of the tube 86. The plate 9S is connected through a :condenser 99 to the iirst grid lilil of the second tube S7. This grid ltlil is biased to its filament lby a resistor lill in series withV a lter 102 consisting of resistor 103 and capacitator lillitL to the positive line 82, while `a lead 104 beween the resistor lill and filter 102 is connected to the negative side of the battery 8l. The plate 16S ofthe tube S7 is connected to a tuned parallel capacitance 166 and inductance 167 network. The inductance ltl' serves as one iside of an output transformer lS Iwhose other coil M9 is connected across output leads `llt) and lll'. In practice, the oscillator illustrated had an output frequency of 360 c.p.s., and the values of some of the parts are shown on the drawings.

The lead llil goes to` one corner ll2 of the bridge ll3 while lead lll .goes to the :opposite corner llfi yof the bridge. The bridge corner :H2 is ,connected to a third corner llS by a variable resistor ll6, while the corner il@ is also connected to the corner llS by a variable resistor Vll7 together with a bypass variable trim condenser 113i.

The leads' 32 and 33 of the moisture unit or probe 3u (or 50 or 60) are connected to respective leads l2@ and l2l. The lead l2@` is connected directly tothe corner ll4 while the lead l2l is connected to the fourth corner 122 of `the bridge 113. The corners llZ and 122 are then connected to eachother .by a variable resistor or rheostat 123. The corner ll is preferably at ground potential, as @by a grounding lead 124 and is connected to the corner 122 by leads l25 and 126 that pass across a Ballentine voltmeter 127 or other similar device. This setup is known as a Bouyoucos bridge.

rPhe-re are, of course, many ways in lwhich the moisture rneter of this invention may be calibrated. One of these is to use saturated salt solutions in sealed asks, which tend to produce well-known values of relative humidity in the atmosphere in the flask, at standard temperatures. A unit 30 is inserted in ia sealed ask containing these sal-ts and the resistance measured across the meter. Table I shows some salts and the humidities they produce at ,21" C. although these data are well known.

9 Table I Percent relative Salt solutions: humidity 21 C. Sodium nitrite, NaNO2 66 Ammonium sulfate, (NH4)2SO4 81 Potassium bisulfate, lil-i804 l86 Zinc sul-fate, ZnSO4.7H2O 90 Sodium acid phosphate, Nfa2HPO4-12H2O 95 Lead nitrate Pb(NO3)2 98 In actual tests solutions of the above salts were placed in the bottom of 100G-ml. Erlenmeyer asks and stored at 21 C. The flasks were sealed with hygrometers 41 suspended from the neck in the atmosphere but not in contact with the solutions tor the calibration. A typical Calibrating curve resulting trom the plotting of the resistance in a hygrometer 41 against these known relative humidities is shown in FIG. 10. Such curves may then be used wherever relative humidity is to be tested.

Numerous variations may'be achieved by varying the construction of the apparatus. Thus, 4a dierent calibration would ybe obtained with a different type of coating solution; yand a different result would be obtained if the wires are spaced different :distanc apart or wound into longer or shorter units. Wires have been wound at various spacings, such 'as five, ten, `four-teen, and twenty turns per inch. As might be expected, units with fewer turns per inch were less responsive and somewhat less accurate. Excellent results were obtained with units wound lat the narrower spacing. FIG. l11 shows the response of two test units both wound at twenty turns per inch tested at 70 F. o-ver a range tot' humidities. However, in one unit the windings were fifteen inches long, and in the other unit they were thirty inches long. The effect of decreasing either the rate of wind or the total length of wire was to shift the curve upward on the resistance scale and to decrease the uniformity of the response.

To indicate the differences caused by the type of coating materials, reference will be made to FIGS. 12, 13, 14, and 15. ln FIGS. 12 and 13 the units were coated with a vinyl acetate copolymer as the hygroscopic medium. All these units were wound lat twenty turns per inch and yteste-d `at various known relative humidities, to give the two curves shown in each graph. In FIG. 13 the solution comprised 25.0% ot du Pont Elvalam a vinyl acetate polymer, dissolved in ethanol, whereas in FIG. 12, 3.5 of the same material was dissolved in a iifty-ifty mixture of ethanol and distilled water. The results of concentration are shown and it will be seen that the curves were different, due :solely to the increased concentration of the coating material.

FIGS. 14 and 15 illustrate the changes made with polyvinyl alcohol in similar situations. In each of these figures two curves are shown, one where the winding was ten turns per inch and another where the winding was twenty turns per inch. In FIG. 14 a 3.5% solution of du Pont Elvanol 20-105, which is a partially hydrolized polyvinyl alcohol, was dissolved in a fiftyality mixture of ethanol and distilled water. In FIG. 15 the units were constructed the same except that 12.0% of Elvanol was dissolved in the ethanol-distilled-Water mixture.

In all the cases shown in FIGS. 12 through 15, the moisture units were dip-coated at room temperatures. The units coated with dilute materials were immersed rapidly and were then withdrawn yat the rate of approximately one inch in ten minutes, a `clock mechanism bein-g employed for precision in this part of the coating openation. With the concentrated solutions of FIGS. 13 and 15 the units were immersed slowly and were thereafter withdrawn by hand over a period of about twenty seconds. The application in all cases was at room temperature, except that the concentrated solution of Elvalran was applied at 45 C. to prevent precipita-tion.

Some further indication of the effect of concentration and dilution'of the Elvanol solution is shown in FIG. 16

where the two are compared on one graph, showing that the concentrated solution resulted in a thicker and more concentrated coating that Ireduced the resistance considerably, making it more sensitive to low relative humidities and less sensitive to high relative humidities. This points to the construction of moisture meters for different ranges of humidities or moisture contents.

It has been mentioned earlier that the polyvinyl alcohols were preferred to the polyvinyl acetates in general. One reason `for this was the relatively quick response obtained by the alcohol solutions, and this is shown in FIG. 17 which graphically displays the time required for typical units to achieve equilibrium. The polyvinyl alcohol achieved equilibrium in about a half hour, while equilibrium was not achieved by the polyvinyl acetate for almost eight hours.

The effect of temperature may be noted. An increase in the temperature decreases the resistance reading. This is common to all resistance meters, because Water decreases in resistance with increased temperatures. Calibration curves may be made at varying temperatures, as has been done, or the temperature gradient can be learned sand applied, 0r measurements may be carried `at standard temperatures. It is quite a simple matter to provide a thermocouple adjacent to the meter wherever it is installed to give the temperature there so that the readings can be properly evaluated.

Some examples will now be given of probes where the permselective membranes themselves are utilized as the moisture-sensitive medium of the hygrometer and where the protective membrane was made by dipping wire- 'found yframeworks in a powdered ion-exchange resin dispersed in a carrier. Vinyl plastics, polyethylene, and synthetic rubber-latices are suitable carriers. Flexible, leaf-like hygrometers may be made by winding parallel wires on a thin sheet, such as l/16 polyethylene or anionic Amberplex, an ion-exchange resin, for example. These assemblies or similar ones may then be covered with a membrane of either anionic or cationic exchange resin, or a combination of both may be used. The membrane may be formed by dipping the assembly into a mixture of one or more ion-exchange resins and a carrier. For this purpose, Ambelplex in powdered =form may be used. For example, a cationic dip mixture may be made by a direct mixture ofthe Amberplex cationic powder in Hyoar 1852, understood to be a polyanylic latex emulsion. Anionic mixtures may be made by rubbing or rolling a partially dry sheet of latex emulsion in powdered anionic resin, to prevent -occulation of the anionic material. A dual mixture may be prepared by taking a partially dry sheet of latex emulsion containing admixed powdered cationic resin and rubbing it with or rolling it in powdered anionic resin to produce a mixture.

A probe comprising only a cationic permselective membrane as both the ionic barrier and the moisture-sensitive film, tested as follows:

Relative humidity* Electrical resistance, ohms 20% 295,000 5 0 170,000 98 3 0,000

A second probe, comprising only an anionic permselective membrane as both moisture-sensitive tilm and the ionic barrier, tested as follows:

Relative humidity- Electrical resistance, ohms 20% 400,000 50% 240,000

A third probe was made using `about 4four feet each of wire electrode wound on a 11/2 x 4" polyethylene sheet 3/16". thick and encased in an inner casing of anionic membriane, which served as the hygroscopic medium. This was then encased in a cationic membrane. Salinity did not appear to afect this meter, which showed a resistance of 440,000 ohms at 20% relative humidity and a resistance of 200,000 ohms at 50% relative humidity.

' Y A Ifourth probe was made comprising both anionic and cationic resins in latex, used as both the ionic barrier and the moisture-sensitive element. A polyethylene core was wound with two wires about eight feet long each and was dipped in ya suspension of about 30% cationic resin in I-Iycar 1852; then anionic resinY was spread over the surface in a thick layer. Then, while the Hycar-cationicresin mixture was still wet, it was dipped again in the mixture of Hycar Aand cationic resin. The device tested as follows:

Electrical resistance, ohms Relative humidity- Little difference was observed between its embodiment in an electrolytic solution and one which was free of electrolytes. There Iwas no shorting out, and no discrepancies traceable to the composition of the solution.

The tests on concrete drying, which are soonV to be discussed, were made using probes similar to the unit 30 shown in FIGS. l through 4. Polystyrene tubing g/s" in diameter and 11/2" long Was threaded with biiilar turns Vfor exactly one inch. Jewelers palladium Wire No. 38

AWG was -used `as described before, and the dilute (3.5%) solution of polyvinyl alcohol (du Pont Elvanol 20-105) was used as the coating agent. A rate of wind of twenty turns per inch for each wire was used. Amberplex ion selective membranes, made by Rohm and Haas, were fused as the'ion selective membranes, these embodying Ithe invention describedV in Patents Nos.f2,681,319 and 2,681,- 320. Y As shown in FIGS. 1 through 4, both anionic and cationic membranes Were used. They were sealed tight with a soldering iron. A plastic sealing compound was applied where the lead ywires 32 and 33 came through the membranes.

Calibration technique available at the test showed that the units were capable of accuracy better than plus or minus .5% moisture, according to the dry Weight of the specimen, but the calibration methods available were not as accurate as the meters so that the limit of accuracy is not known; the meters are believed to be very sensitive indeed.

IIt is well known that the amount of moisture present in hardened concrete has important eiects upon all of its significant properties. 'If the concrete is not moist enough at early ages for hydration to be carried on efliciently and completely, the strength, durability, and elastic properties of the concrete are age or expansion resulting from changes in moisture conftent may produce cracks or excessive beam and slab deection or other undesirable effects. In hydraulic structures concrete mayA be in direct contact with aggressive water, which could cause swelling and disruptive pressures. Changes in moisture content may result in changes of from 3% to 6% of the weight of the structure, which may be important where gravity forces are torbe considered.V The success with which paint, linoleum, and other covering materials can be applied to concrete depends upon the extent to which the concrete tends to take up or to release moisture.

Three types of Y nized: (l) water is held by chemical bonds through hydration (chemically combined water); (2) water is held by physically developed surface forces (physically held water or solvation water); and (3) other water is free to leave the mass Without affecting the cementing value of the paste and is beyond the inuence of surface forces (capillary water). Physically-held and capillary water are the ones which cause shrinkage or swelling of' the mass by a change in this water. This is the water which may freeze and by alterna-tive freezing and thawing over many cycles may disintegrate a concrete mass.

decreased. Shrink- Y water association in concrete are recog- Ybeing defined as water lost during drying to constant weight at 105 C. Thus, a total moisture content may be determined by drying a sample to constant weight at 105 C. and comparing that weight with the weights before drying or weights during drying, or weights afterwards if Water is added subsequently. A change from the evaporabie to non-evaporable water due to further hydration does not effect any weight change. n

For the purposes of testing the effectiveness of these moisture meters, several units as described were placed in mortar specimens at the time of casting with the leads 32 and 3K3 projecting out therefrom. These mortar specimens were initially cured,l different specimens for each test condition being stored at two different conditions of temperature and relative humidity. For each specimen measurements were made of weight loss and corresponding resistance of the moisture unit, at intervals of a few days throughout the total test period. For comparisons` and Ydetermination of moisture content, mor-tar specimens were also cast Without the moisture units and were stored at F. and 20% relative humidity. At various agesvthese specimens were dried to constant weight at C. and the weight loss, expressed as a percentage of the dry weight of the specimen, was termed the l was 0.50 by weight, providing a somewhat dry mix capable of placement through vibration to produce a mortar without large voids. The mixer employed Was like that in'ASTM C 30S-55T and comprised blending the dry `material for the `first thirty seconds, adding Water for the next thirty seconds while continuing to mix, and then mixing additionally for one more minute.V The mortar was then placed into forms and vibrated by a vibrating table. The mortar was cured in the forms for one day at 70 Fand at 95% relative humidity. Then the forms were stripped from the samples, and the samples were cured one day at 70 F. and 100% relative humidity. Thereafter half of the samples of each group of specimens Was stored at 70 F. and 50% relative humidity, while the remaining specimens were stored at 100 F. and 20% y relative humidity.

FIG. 19 shows curves plotting weight loss against drying time of specimens stored at these two conditions. Of course, the specimens stored at the higher temperature and lower relative humidity lost moisture much more quickly than did the other specimens. It may be noted that both curves are logarithmic, indicating a high rate of loss at early stages. A scatter obtained when compared with the drying weight method indicated an accuracy of the meters of better than plus or minus .5%.

Companion specimens'were stored in a leaching tank to observe the effect caused by the leaching action of circulating distilled water. Such water withdraws the ionic salts present and enables evaluation of the effectiveness of the ion-selective membranes in preventing ionic action from affecting the measurements. 'The salinity of the leaching Water was determined at intervals by appropriate measurements, and water ywas replaced whenever the concentration of salts tended to value.

FIG. 20 shows that changes rin salinity hadno effect on approach a constant the meters where the probe was enclosed in ion-selective the permselective membranes.

For a given water content expressed as percent of dry weight, the greater the density of the material, the more water there will be per unit volume, so that calibration in terms of water per unitvolume may be better for many uses than of the percent of dry material. The meters of this invention are capable of such use and calibration. This indicates that when water content may be known, the density of the material may be determined so that the meters of this inventionmay be used as density meters.

To those skilled in the art to wh-ich this invention re lates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing `from the spirit and scope of the invent-ion. The disclosures land the description herein are purely illustrative and are not intended to be in any sense limiting.

, We claim:

l. A moisture meter probe whose response is substan` tially unchanged by the presence of ionizable material in an environment whose contained moisture is to be measured, comprising a pair of spaced-apart electrical con* ductors; a hygroscopic medium extending between the conductors; water-vaporpermeable ionselective membrane covering saidmedium and conductors yfor isolating said medium and conductors from ions that affect the conductivity of moisture, and an electrical lead from each said vconductor extending out trom said ion-selective means and sealed therethrough.

2. A moisture-meter probe whose response remains substantially unchanged by ionizable material present in an environment whose contained moisture is to 'be measured, comp-rising a pair of spaced-apart electrical conductors; a hygroscopic medium of the type that comes to an equilibrium moisture content with respect to its surroundings, iilling the space between the conductors; ionelective membrane for passing water vapor to said medium while preventing passage of ions lof said material covering said conductors and said medium; and electrical leads from each said conductor extending out from saidr ion-selective means and sealed thereto.

hygroscopic lilm that comes to an equilibrium moisture content with respect to its surroundin-gs, covering said wires and the space between said wires; a pair of ionexchange resin membranes, o-ne cationic and one anionic, completely encasing said wire, film, and core; and electrical leads sealed in said membranes and `connected to said wires and extending outside said membranes.

9'. A moisture meter probe including in combination a core having a plurality of groups of bilar conductors on said core, each pair of conductors being spaced apart in its group, the groups being lof different length; a hygroscopic lm that comes to an equilibrium moisture contentl with respect to its surrounndings, covering said Wires and the space between said Wires; ion-exchange resin membranes completely encasing said wires, film, and core; and electrical leads sealed in said membranes and I connected to said wires and extending outside said 3. The probe of claim 2 wherein said ion selective means comprises a permselective membrane.V

4. The probe of claim 2 wherein said ion selective means comprises a cation selective membrane and an anion selective membrane placed back to back.

5. The probe of claim 2 wherein said hygroscopick medium is polyvinyl alcohol.

6. The probe of claim 2 wherein said hygroscopic medium is a portion of said ion-selective means.

7. A moisture meter probe whose response is substantially unchanged by ionizable material present in an environment whose contained moisture is to ibe measured, comprising a pair of spaced-apart electrical conductors; a hygroscopic medium of the type' that comes to an equiiibrium moisture content with respect to its surroundings filling the space between the conductors; a casing means for passing Water vapor and barring passage of liquid water salts in solution and of ions affecting the conductivity of water, covering said conductors and said medium; and electrical leads from each `said conductor extending out from said casing and sealed therethrough.

8. A moisture-meter probe including in combination a dielectric core having -biiilar helical grooves spaced apart at an even interval and at a constant pitch; a pair of palladium wires, one wound in each said groove; a

membranes.

10. lA moisture-meter probe including in combmation a dielectric sheet; printed biiilar conductors thereon spaced apart at an even interval and at -a constant pitch;

a hygroscopic iilm that comes to an equilibrium moisture content with respect to its surroundings, covering said conductors and the space between them; a pair of ion-exchange resin membranes, one cationic and one anionic, completely encasing said sheet, conductors, and coating; and electrical leads sealed in said membranes and connected to said conductors and extending outside said membranes.

l1. A moisture-meter probe including in combination a dielectr-ic cylindrical core having two groups of bilar helical grooves spaced apart in each group at an even interval and at a constant pitch, one group being llonger than the other; two pairs of palladium wires, one wound in each said groove; a hygroscopic film that comes to an equilibriummoisture content with respect to its surroundings, covering said wires and the space .between said wires; ion-exchange resin membranes completely encasing said wires, film, and core; and electrical leads sealed in said membranes `and connected to said wires and extending outside said membranes.

12. A moisture meter probe including in combination a dielectric core having bilar helical grooves; `a pair of noble metal wires, one Wound in each said groove; a hygroscopic iilm that comes to an equilibrium moisture content with respect to its surroundings, covering said wires and the space between said wires; a .pair of ion-exchange resin membranes, one cationic and one anionic, completely encasing said wire, iilm, and core; and electrical leads sealed in said membranes and connected to said wires and extending outside said membranes.

13. A moisture meter probe including in combination ya dielectric sheet; bilar conductors spa-ced apart thereon;

. a hygroscopic iilm that comes to an equilibrium moisture References Cited in the tile of this patent UNITED STATES PATENTS 2,543,384 Squier Feb. 27, 1951 2,722,586 Stearns et al Nov. l, 1955 2,740,032 Bouyoucos Mar. 27, 1956 2,793,526 Dalglish May 28, 1957 2,937,524 Gregor May 24, 1960 OTHER REFERENCES Treatise on Physical Chemistry, Taylor, vol. 1, 2nd edition, D. Van Nostrand Co. Inc., New York, 1931, relied lon pages 406-414.

UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 3,105,214 September 24, 1963 William FL. Blythe et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 2, for "to produce" read do produce line 17, for "on" read in line 18 for "at" read as column 6, lines 1 and 2, for "anion-permeable" read anionc-permeable line 29, for "dried" read drier column 11, line 18, for "embodiment" read embeddment Signed and sealed this 2nd day of June 1964 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A MOISTURE METER PROBE WHOSE RESPONSE IS SUBSTANTIALLY UNCHANGED BY THE PRESENCE OF IONIZALBE MATERIAL IN AN ENVIRONMENT WHOSE CONTAINED MOISTURE IS TO BE MEASURED, COMPRISING A PAIR OF SPACED-APART ELECTRICAL CONDUCTORS; A HYDROSCOPIC MEDIUM EXTENDING BETWEEN THE CONDUCTORS; WATER-VAPOR-PERMEABLE ION-SELECTIVE MEMBRANE COVERING SAID MEDIUM AND CONDUCTORS FOR ISOLATING SAID MEDIUM AND CONDUCTORS FROM IONS THAT AFFECT THE CONDUCTIVITY OF MOISTURE, AND AN ELECTRICAL LEAD FROM EACH SAID CONDUCTOR EXTENDING OUT FROM SAID ION-SELECTIVE MEANS AND SEALED THERETHROUGH. 